Transparent polyester carbonated beverage containers are in wide-spread use around the world and have largely replaced prior art glass containers for soft drink beverages. The plastic containers are substantially lighter in weight, and shatter resistant. The polyester most commonly used, polyethylene terephthalate (PET), provides superior clarity, recyclability, and ease of manufacture at a competitive price.
Despite substantial uniformity in the material used to make plastic carbonated beverage containers, each beverage manufacturer would like to distinguish the visual appearance of their bottle from competitors' bottles. One way to accomplish this is by applying a distinctive label to the container. Another way is to customize the contour of the container itself to provide a distinguishable visual appearance that consumers learn to recognize. Ribs are one feature that can be utilized in almost endless variations, to customize the look of a container. The ribs may be singular, plural, extend radially inwardly or outwardly with respect to the container circumference, and otherwise form patterns which distinguish the container. The ribs may also provide structural reinforcement to prevent buckling of the container.
One problem with prior art contour ribs in pressurized containers is their tendency to "creep" (move) under pressure. This produces an increase in container volume and an undesirable pressure loss in the carbonated beverage. The problem of creep is illustrated for two prior art containers in FIGS. 1-4. Both are representative of known transparent PET carbonated beverage containers of 1/2-liter volume, one having internal ribs and the other external ribs.
FIG. 1 shows prior art container 10 having ten vertically-disposed ribs 12 in panel 14 and shoulder 16 sections. The ribs extend radially inward (are recessed) from container circumference 18, as shown in FIG. 2 (a cross-section through the panel portion 14). The solid line in FIG. 2 is the panel circumference 18 after blow molding, and prior to filling with a carbonated beverage. The ten relatively large-radius ribs 12 are symmetrically disposed about the panel circumference 18, which is defined by radius R.sub.1 (radial distance from vertical centerline CL of the container). After filling, the panel undergoes creep in a radially outward direction, such that the originally inwardly extending ribs tend to flatten out about the circumference (i.e., the ribs are substantially eliminated) and the panel forms a substantially cylindrical panel circumference 18' having a radius R.sub.2, which is somewhat greater than R.sub.1. This is clearly undesirable from the viewpoint of the beverage company for at least two reasons. First, the container is losing a significant contour feature which is intended to distinguish this company's container from other containers in the marketplace. Secondly, the increase in container diameter produces a resulting volume increase in the container, which leads to a lower pressure in the headspace, i.e., the volume of pressurized air above the liquid in the top end of the container. This reduction in headspace pressure causes gas in the pressurized liquid (carbonation) to leave the liquid and enter the headspace, resulting in an undesirable drop in the carbonation level. The beverage company would like to maintain tight control over the carbonation pressure in order to deliver an optimum product to the consumer. In this regard, the company establishes a shelf life for its product, which specifies a maximum loss in carbonation pressure over time. In effect, the volume increase due to expansion of the ribs reduces the shelf life of the product. This increases the cost to the manufacturer in that he now must either sell the product in a shorter time period or replace expired product with fresh product on the retail store shelves.
FIG. 2A is an enlarged fragment of the panel cross-section, showing more clearly the original outer panel circumference 18 at R.sub.1, and the enlarged outer panel circumference 18' at R.sub.2 after filling. The angular extent A between ribs 12 is defined as a circumferential distance in degrees between the center points of two adjacent ribs. Each rib is defined by a relatively large radius R.sub.3, e.g., 0.100 to 0.200 inches (0.254 to 0.508 cm). A smaller blend radius R.sub.4 smoothly connects the opposing edges of the rib to the container circumference 18.
FIGS. 3-4 show an alternative prior art container 20 which is the same as the first container (of FIGS. 1-2) but wherein vertical ribs 22 extend radially outwardly (protruding), rather than inwardly. Note that like features are given similar reference numbers with respect to the first container, but in a range of 20-29 as opposed to 10-19. In this embodiment, the original panel circumference 28 is at R.sub.10. After filling, the panel circumference 28' has experienced a net overall radial increase to R.sub.11, with the ribs again flattening out about the circumference. Again, each rib has a relatively large radius R.sub.12, and a relatively small blend radius R.sub.13.
Another significant problem caused by rib movement in prior multilayer pressurized containers is delamination. Often, a manufacturer would like to provide a multilayer wall in some portion or all of the container, in order to influence the overall cost of materials, thermal resistance, barrier properties (e.g., loss of CO.sub.2 and/or ingress of oxygen), processibility, etc. In particular, smaller sized containers, having a high surface area to volume ratio, often cannot be produced with an acceptable shelf life unless a barrier layer is included. However, in multilayer pressurized containers with rib contours, when the ribs move under pressure (creep) so as to substantially flatten out about the panel circumference, this often produces delamination (separation) of the layers. Layer separation is undesirable as it may lead to loss of transparency, structural weakness, loss of barrier properties, etc. Layer separation can be a particular problem in multilayer containers without chemical bonding or adhesives to bind the layers, e.g., recyclable containers wherein relatively weak diffusion or hydrogen bonding maintains the layer structure during use, but enables ready separation when cut (during the recycling process).
Thus, there is need for a pressurized container for carbonated beverages and the like which can be customized, but which avoids the above problems of pressure loss and delamination.